The present invention relates to a method of chemical decomposition by which the volumes of spent radioactive ion-exchange resins (hereinafter sometimes referred to as waste resins) originating from atomic energy facilities can be reduced.
Ion-exchange resins are extensively used in many applications such as purification of water, treatment of wastewaters, and separation of various elements. They are also used in large quantities in the field of atomic energy for the purpose of purifying cooling water in nuclear reactors and treating liquid wastes. Therefore, treatment and disposal of spent waste ion-exchange resins containing radioactive substances has been a serious concern in this field.
The method in common use in the technology of disposal of radioactive waste ion-exchange resins is to dehydrate the resins, solidify them by incorporation in cements, plastics, etc., place the solidified wastes in containers, and store them for a prescribed number of years, often almost perpetually. However, the waste resins treated by this method are not reduced significantly in volume and have posed substantial problems in the area of waste storage and management.
As described above, no really satisfactory method for treating or finally disposing of the variety of solid wastes that result from the operation of nuclear power plants has yet been established. One of the serious problems that remain to be solved is how to reduce the volume of ion-exchange resins that are discarded after they have been used in the purification of liquid media.
Several methods, including combustion (incineration), pyrolysis, and acid decomposition have so far been proposed as techniques for reducing the volumes of waste ion-exchange resins, but none of these have proved to be a complete solution to the problem. The combustion method has the advantage of achieving rapid treatment but, at the same time, it requires complicated off-gas lines for handling dust and tars, and/or produces volatile radioactive compounds. The last-mentioned problem is absent from the pyrolysis method, but, on the other hand, it yields high residual contents of carbonaceous materials, and still requires complicated flow systems as in the case of the incineration method. In the acid decomposition method, up to about 90% of the spent ion-exchange resins can be decomposed by successive treatments with concentrated sulfuric acid and nitric acid at a temperature of about 260.degree. C. Although this method is free from any of the problems associated with the first two methods, it has the disadvantage of generating NOx and SOx. Furthermore, the reaction vessel must be made of an expensive material such as tantalum that is capable of withstanding the extremely high temperatures employed. As a further problem, the volume of the waste resins being treated cannot be reduced to the desired extent, since large quantities of salts form during neutralization of the reaction solution.
In order to avoid this problem, a method of decomposing waste resins at about 100.degree. C. using hydrogen peroxide and an iron catalyst has been described in Japanese Patent Application (OPI) No. 1446/82 (the term "OPI" as used herein means an "unexamined published Japanese patent application"). This method readily achieves up to 95% decomposition if the waste resin is a cation-exchange resin, but the decomposition of an anion-exchange resin is no higher than 90%. To overcome this disadvantage, it has been proposed that a combination of iron and copper ions be used as a catalyst when the waste resin is decomposed by oxidation with hydrogen peroxide (Japanese Patent Application (OPI) No. 44700/84). This approach achieves at least 95% decomposition of anion-exchange resins, but if the amount of feed (i.e., anion-exchange resin) is increased, organic sludge containing iron and copper ions will form. Furthermore, the decomposition of waste resins by this method has been found to be highly dependent on the pH of the reaction solution, notwithstanding the previously held view that good decomposition is achieved within the pH range of 3 to 11, with particularly good results being attained in the neighborhood of neutrality. If the organic sludge is formed in a large quantity, it will be accumulated in the reaction vessel (reactor) or pipes to form "secondary" wastes which require another treatment, and may even cause a problem with transportation.
Other problems exist in the method of decomposing waste resins with hydrogen peroxide using iron and copper ions as catalysts. First of all, the reaction rate is very slow (at least one to two hours is necessary to convert the waste resin to inorganic matter), and a reactor of large capacity is required. Secondly and because of this slow reaction rate, decomposition must be performed under fairly H.sub.2 O.sub.2 -rich conditions. Since the running cost of this method is essentially determined by the amount of hydrogen peroxide used, it is important both technically and economically to achieve decomposition with the least possible amount of hydrogen peroxide used.
The prior art technology also has another problem that has to be solved before it can be employed in practical applications, viz., leakage of radioactivity from mechanical seals in the agitating and mixing apparatus used for achieving accelerated decomposition reaction.